White-throated dipper (Cinclus cinclus), a bird associated with high quality streams. Image credit, Thomas Kraft.A new study in the journal Environmental Management shows that riparian bird communities can function as indicators of early stream and riparian degradation.

Assessments of stream integrity usually look at the organisms that live in the water - particularly macroinvertebrates such as crustaceans and insects. This makes sense because these are organisms directly affected by water pollution.

However, according to Stefano Larsen and fellow study authors, evaluating stream quality by looking at birds may have some advantages. Riparian bird species are directly affected by terrestrial and aquatic processes and thus reflect both systems’ functionality.

The response of birds potentially precedes changes in the aquatic systems, so birds may be able to give an early warning about impending stream degradation. Furthermore, bird surveys may be more cost-effective than traditional stream assessments.

To test the approach of looking at avian communities as bioindicators, the team sampled birds at 37 sites along 5 streams located in the Tyrrhenian seaside of central Italy. The sites are subject to varying degrees of human disturbance including agricultural and residential development.

The researchers also surveyed macroinvertebrates in the streams to construct a direct, formalized score of stream quality (i.e, the Italian Extended Biotic Index). They then analyzed how stream quality and level of disturbance in the surrounding landscape related to the presence/absence of individual bird species and the overall tropic structure of avian communities.

The study found that the overall structure and function of bird communities was a solid indicator of stream health. Species richness and diversity were directly related to the water quality index.

Insectivorous birds and those nesting and feeding on trees were more common in reaches with the highest water quality scores and lowest levels of landscape disturbances. Meanwhile, degraded riparian areas were characterized by larger species that tended to nest on grass or shrubs and feed on plant material.

However, the researchers found that the presence-absence of individual species showed a weaker relationship with stream quality. The authors write,

"Our results have shown that, in riparian areas, diversity and trophic measures of avian communities are good candidate metrics for an integrated stream bioassessment; such measures could serve as indicators of riparian habitat alterations and act as an early sign of stream degradation."